1. Field of the Invention
The present invention relates to a sensor and more particularly but not exclusively, to a biosensor.
2. Related Art
Biosensors are used for determining the presence and/or amount of an analyte in a sample. Typically a biosensor comprises a bio-component (e.g. enzyme) which is specific for the analyte to be determined and which interacts therewith to produce a detectable change indicative of the presence and/or amount of the analyte in a sample. A particular example of biosensor comprises an enzyme electrode system whereby the catalytic reaction of suitable redox enzyme on analyte produces an electroactive species which is detected electrochemically. A glucose sensor using glucose oxidase as the bio-component is a specific example of an enzyme electrode system. Here glucose is oxidised by oxygen via glucose oxidase to produce gluconic acid and hydrogen peroxide. the latter being detected electrochemically. Systems based on oxidase enzymes are particularly well suited to biosensors because the oxygen acts as the mediator (i.e. to shuttle electrons to the electrode from the enzyme active site) and no additional reagents are required (i.e. it is a reagentless system). However, not all analytes have corresponding oxidase enzymes therefore limiting the number of substrates that can be determined using this system. Dehydrogenase enzymes are more diverse than oxidases and hence enable that a wider variety of analytes can be measured by biosensor systems. A key disadvantage of dehydrogenase enzymes, however, is that oxygen cannot act as a mediating (electron accepting) species and instead soluble nicatinamide adenine dinucleotide (NAD(P).sup.+ /NAD(P)H) cofactors are required to act as electron acceptors/donors. Thus, biosensor systems based on dehydrogenase enzymes are not reagentless which restricts practical application.
It is known from European Patent Specification No. 402917 to provide a biosensor employing a thin surfactant polymeric electrically conducting layer to which may be bound members of specific binding pairs. Binding of an analyte to the specific binding pair member layer changes the electrical properties of the layer to enable the detection of the analyte. A DC voltage is applied between a pair of electrodes bridged by the layer, and changes in the voltage are measured. Alternatively, electrical alternating current measurements may be used for filtering out background noise due to non-specific matrix effects. Unfortunately, the sensitivity of such biosensors is limited as the measured changes in the electrical properties of the layer are relatively small.